Boost unit, power conditioner, photovoltaic system, program, and voltage tracking method

Occasionally an input voltage at a boost circuit (20a,20b) at which combined power of at least two of photovoltaic strings (200a,200b,200c) is maximized cannot be tracked by MPPT control. A voltage identification unit (inside control device 100) identifies, when a change in input power or output power of a first boost circuit (20a) satisfies a predetermined condition, the input voltage at the first boost circuit at which the output power of the first boost circuit is maximized at timing corresponding to the condition. A voltage tracking unit (inside control device 100) tracks, when the voltage identification unit identifies the input voltage at the first boost circuit at which the output power of the first boost circuit is maximized, the input voltage at the first boost circuit at which the output power of the first boost circuit is maximized while changing the input voltage at the first boost circuit with the input voltage based on the identified input voltage at the first boost circuit as a start point.